i STRUCTURE AND PHYSIOLOGY OF ANIMALS 37 



the blood and thrown out of the body by a distinct set of organs 

 called renal organs, or organs of urinary excretion. The 



form of these organs varies greatly in the different groups; 

 in many cases they are more or less intimately connected with 

 the genital system. 



In place of the simple contractions and extensions of the proto- 

 plasm which constitute the only movements of Amoeba, the higher 

 animals are capable of complex and definite movements. These 

 are brought about by the agency of a set of organs termed the 

 muscles. A muscle is a band or sheet of muscular fibres 

 endowed in the living state with the property of contractility, by 

 virtue of which, when stimulated in certain ways, it contracts in 

 the direction of its length, becoming shortened, and, at the same 

 time, thickened (Fig. 25). The extremities of the muscle are 



FIG. 25 . Bones of the human arm and fore-arm with the biceps muscle, showing the shortening 

 and thickening of the muscle during extraction and the consequent change in the relative 

 position of the bones viz., flexion of the fore-arm on the upper arm. (From Huxley's Physiology.) 



frequently composed, not of contractile muscular fibres, but of a 

 form of strong fibrous connective tissue the tendon of the muscle. 

 The ends of the muscle are usually firmly attached to two different 

 parts of the jointed framework or skeleton, external or internal; 

 and, when the muscle contracts and becomes shortened, these two 

 parts are drawn nearer to one another. 



In all but the most lowly-organised animals there is a system 

 of organs the nervous system by means of which a communi- 

 cation is effected between the various parts of the body, enabling 

 them to work in harmony, and by means of which also a communi- 

 cation is established between the organism and the external 

 world. The two essential elements of the nervous system the 

 nerve-cells and nerve-fibres have a regular arrangement which 

 varies in the different animal types both as regards structural 

 details and the relations borne to the other systems of organs ; 

 but there are to be recognised two chief parts or sets of parts- 

 the central and the peripheral. 



